METHODS AND EQUIPMENT FOR LONG-WAVE INFRARED SPECTROSCOPY, 39 PP

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• Sequence Number: 2
• Publication Date: August 7, 1958
• Content Type: REPORT

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(Translation frorrittus sian)
From:
? Uspekhi Frz. Nauk 62,
No. 2, pp. 159-186.. ( ji c
New Instruments and Methods of Measurement
- METHODS AND EQUIPMENT *
r .
FOR. LONG-WAVE INFRARECTROSCOPY
N. G. laros1avskii
I. INTRODUCTION
In the present work the whole infrared* range of-the optical spectrum,
extending from the red limit of the visible spectrum towards the longer wave-
lengths and overlapping the region of millimeter radio-waves, is assumed to
be divided into three regions [1, 4, 5]: a) a:short-wave region bounded by the
wavelengths 0.75 and 2.5p (13,300 - 4,000 cm-1), b) a medium-wave region
Z.5- 50p (4,000 - 200 cm-1) and c) a long-wave region covering a large range
of wavelengths from 50p (200cm-1) to 1000p (10cm-1) and more.
? Within the short-wave portion of the IR spectrum, i.e. in the 'very near"
IR region, are situated the bands corresponding to the overtones and compo-
nent frequencies of the fundamental vibrations of molecules, as well as lines
and bands of the electron-vibrational spectrum of atoms and molecules.
Thernedium-wave range (2.5- 50p) contains mainly bands corresponding
to the fundamental vibrational frequencies of light molecules and individual
atomic groups. (This spectral region is therefore sometimes called the region
of fundamental vibration frequencies.)
Methods- of measuz4ment of IR spectra in the wavelength region from 0.75
to 50p do not present much difficulty at the present time and they find wide
practical application in resdarch and factory laboratories. For studies in the
short-wave IR region, which is most accessible to experiment, spectrometers
*For brevity the word "infrared" will henceforth be replaeed by the abbrevia-
tion IR.
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with glass or quartz prisms, are usually employed along with standard tungs-
ten incandescent lamps in glass bulbs as sources and photocells or photo-
resistors as detectors.* Spectrometers designed for work in the mediuni-
wave IR region make use of prisms Mdde?from crystals of alkali halide salts, ,
including the cesium iodide crystal which is transparent up to, 50 - 55?, and
have as sources Nernst or Globar filaments (IKR-1 filaments, silicon carbide
rods) heated by an electric current; heat detectors are employed, Metallic
and semi-conducting thermoelements and bolometers, as well as optico-
acoustic (pneumatic) receivers. For obtaining high resolution with these
instruments, as also with spectrometers for the short-wave IR. region, the
prisms are replaced by diffraction gratings - echelettes with a large number
of lines.
Techniques and instruments for producing and recording IR spectra-in
the 0.75- 50? region have been well developed and are described fully enough
in home and foreign surveys in the literature [I-5). ?
In the long-wave IR region, i.e. in the wavelength range from SO to 100011
and more, are located absorption and transmission bands corresponding to
???
the variation of the rotational energy of as and vapor molecules, as well as
low-Lrequency vibrational bands of heavy molecules, radicals and molecular
complexes. In addition this region contains various component (difference)
frequencies of fundamental vibrations of various molecules,,anclalso inter-
molecular vibration frequencies.
Investigation of the long-wave IR spectra of different gaseous substance,
opens up the passibility of a direct study of their molecular structure (deter-
`mination of molecular moments of inertia, interatomic distances and other
molecular constants).
Investigation of the low frequencies of intra- and inter-molecular vibra-
tions in solid,and liquid bodies is essential to the study of crystal structure,
the nature of the liquid state and molecular interactions. There are 'also other
fields of application for long-wave infrared spe4roscopy and this technique
is now being used to study the optical and electrical properties of semicon-
ductors and dielectrics in the region of longer waves [40, 421.
The application of the method of long-wave spectroscopy is apparently
also of great value in the study of light-scattering by dispersed bodies ,
(powders) and its dependence on the size of the scattering particles, their
form and spatial, distribution. The purpose of this investigation might be to
establish the laws governing scattering of radiation by arlificiatly modelled
* Hence the short-wave IR region is sometimes named the 'photoelectric"
region.
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dispersed media with accurately predetermined geometrical parameters,
comparable with the wavelength.
Finally, the recently established bands in the terrestrial atmo?phere,
transparent for electromagnetic vibrations with wavelengths of about 1500p
(1.5inrn) give us grpunds for believing that long-wave IR spectroscopy will
fixeren application also in astronomical spectral investigations, for example,
In determining the temperature of planets [6].
?
In spite of the fact that long-wave infrared (*heat") radiation was dis-
covered exactly 60 years ago [II), it was a lonf time before it was applied in
snelecular spectroscopy. This was due to the gkeat experimental difficultieu
connected mainly with the very low energy in the long-wave IR spectrum, and
also with the Lack of high-sensitivity recording and detecting devices. It is
only in recent times, thanks tbk.the progress in techniques for detecting weak
radiation and methods of amplifying weak currents, as well as to the develop-
ment of new large-aperture diffraction gratings (echelettes) and aspherical re-
flecting optics of large size, that the long.wave IR region has begun to be
applied in increasing degree to the solution_of various problems of physics
and chemistry. Nevertheless, the number 3f works devoted to studies in this
spectral region is still very small.
In the present survey, which makes no claim to completeness, an attempt
has been made to systematise the information relating to methods o( long-
wave IR spectroscopy and the equipment dveloped in recent times both aft
home and abroad (or producing and recording spectra in the waveleng4J
region up to 1600p.
IL METHODS OF SOLATiNG LoON/G-WAVE INFIRARED RADIATION
The following methods may be used for obtaining long-wave infrared
monochromatic radiation: 1) the method of focal isolation using quarts lenses,
2) the method based on total internatriflection. 3) selective reflection from
crystals (method of *residual rare) and 4) monochromatisation by means of
diffraction gratings.
The first three methods possess low resolving power, yet they have
distinite advantages over the fourth in view of their simplicity and the great
ergy of long-wave radiation obtained. On the other 'hind, these methods are
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unsuitable in cases where high resolution is required, e.g., in studying the
rotational spectra of molecules. In these cases it is essential to monothrom-
atize the radiation by using diffraction gratings.
1. Method of quartz lenses
The method of focal isolation of long-wave IR emission using quartz
lenses was first carried out by Rubens and Wood [7] irk' 1910.* The method is
based on the fact that quartz which is transparent in the long-wave region
possesses a higher refractive index (of the order of in the region of
ti
Fig. 1. Method of quartz lenses [7].
wavelengths greater than 50j,. while in the short-wave and visible spectral
region its refractive index is approximately equal to 1.5. ?
Fig. 1. depicts the rriain scheme of an assembly for isolating long-wave
emission by the method of quartz lenses. The quartz lens L1 refracts the
long-wave emission from the radiation source A more strongly than the
short-wave emission. The diaphragms of black paper DI and the metal
screen Dz with aperture ensure that only the long-wave emission falls into
the space behind D For further increasing the hoMogeneity of the radiation,
a second lens Lz repeats the action of the first lens. This is necessary to
remove the short-wave radiation scattered at the surface of the lens Li and
to focus the long-wave radiation on the detector M.
The dimensions of the apparatus of Rubens and Wood were as follows:
the diameters of the effectivcaperture of both quartz lenses was equal to
7.5 cm, their thickness at the edges was 0.3 cm, and in the middle, 0.8 cm;
*Before Rubens and Wood this method was used for achieving monochromatic
ultraviolet emission by Lenard [8] who should be consi.dered as the inventor
of the method of quartz lenses. The so-called focal monochromators for the
ultraviolet region which are still in use at the present time [9], are based on
this method.
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the focal length of the lenses for visible rays was 27.3 cm, and for long-wave
IR radiation, approximately 12 cm. The diameters of diaphragms 13 and Ds
were equal to 15 ram, and diaphragm DI ? 5 mm.
? With the total thickness of quartz used in the apparatus there is no trans-
mission at all up to 80p, while at 95p the transmission io only 20% of the inci-
dent long-wave radiation and with increase of wavelength the transmission
increases. In view of this the curve Of spectral distribution of energy falls
rather steeply towards the shorter wavelengths, and considerably more
slowly in the long-wave direction.
For measurement of the wavelengths isolated by the apparatus, which
depend on the relative positions of lenses, source and detector, an interferom-
eter consisting of two quartz plates is placed in the path of the rays at Da. A
glass plate C. opaque to long-wave radiation, serves as a shutter during
measurement.
The method of quartz lenses, as well as the method of 'residual rays'
described below, were widely applied in early studies on the transmission
and reflective power of various materials in the region of longer wave-lesgths
[14, 15]. On account of the very low resolution, however, it cannot be used
for investigating the rotational spectra of molecule.'.
2. Method of total internal reflection
In 1928 lentzsch and Laski [101 suggested a simple method of isolating
any spectral portion in the region of wavelengths 80 ?100?. As is known,
total internal reflection of radiation does nol occur exactly at the boundary
between the two media, but in some region of the second medium, having a
depth of the order of the wavelength. U another plate made from the essential
of the first medium is placed in the second medium close to the reflecting
A
surface, then part of the radiation penetrating into the second medium passes
into this plate and is not returned in the reflected beam. The limiting width
of the air gap for which radiation of a given wavelength still reecho' the
second plate dependS. on the wavelength and increases with th?Iatter. By using
this fact it Is possible to construct an instrument, a diagram of whichls shown
in Figure 2. A cube 1, made from material transparent to long-wave radiation.
stops the short-wave radiation, deflecting it in the direction 1; the remaining
longer-wave radiation is again divided at cube It which has a greater air gap.
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a?
.1m=MMIAM?
Fig. 2. Method of total internal reflection [10].
. ? .
into two portions, the long waves being transmitted in the direction 3, while
?- the shorter are daflectod in direction 2, where we obtain an approximately
? - monochromatic beam, thq degree of moriochromatization and mean wave ength
P of Which depends on the thickness of the air gaps of cubes I and II. ?
_
This method for producing monochromatic long-wave emission, however,
_
? ?
- did not find wide application because of the small aperture, lack of sufficiently
"- transparent Materials for tha long-wave IR. region and the necessity for
_mechanical (tdjurtmant of the prisms of both cubes so that the small air gap
_ ??
.
?
?
? 'botween them is accurately fixed.
?
?. 3:Alethod of 'residual ray..'
- The most commonly used Method of isolating sufficiently narrow regions
?
:?????-? ????'1,. - 'Of long-wave emission is the method of ?residual rays," which is based on
? ' .theproporty of crystals to selectively reflect (and absorb) radiation in the
, - ?
? - regions of anomalous di,apersion, i.e. close to the characteristic vibration
frequencies of the crystalline lattices, where the so-called "metallie"
tionis.observad. This method, proposed Is early as 1897 by Ruben. and
'Nichols [11], is still applied in various investigations at the present time [12].
-
_The principle of the *residual rail; method is the following. Let r be
? ? the reflecting power Of a given substance in the region Of "metallic" 'reflec-
tion. Then
?
tR, 'Oz.: ? ?
? r - + ?Ixa
414'. (s+ If + al xl
?
from unity, since in the region of anomalous dispersion 2n
.47 comparison with AO + na +1. Further, let p be the reflecting
- ?
tpoWeeoi a substance in the region of normal dispersion, in which Fresnel's
stili hail; i.e.
? ? R: ?
?
6
?
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- 1)2
P
(I) ? 1)2
--abefrIMECte?
c.710a1MittrirW,''' t
We will denote by i, the ratio of intensities in two portions of,.the continuous
spectrum, one of which lies in the region of anomalous, the Other in the
region of normal dispersion. After k-fold reflection from the surface of the
given substance this ratiO becomes equal to ik =i(r/p)k. If, for example,
i =1, r =0.9 and p =0.45, then after fourfold reflection ik r- 24 = 16, whereas
after a single reflection ik is equal to only two. Thus, by means of multiple
reflections it is possible to isolate from the dpectrum a fixed interval of
wavelengths, namely that lying close to the characteristic vibration frequency
of the crystal lattice of the substance. This explains the term "residual rays,"
applied to radiation isolated by this method.
For obtaining "residual rays," only the greatest reflection maxima of
crystals can be used, since otherwise the intensity of long-wave radiation, of
which rk serves as a measure, is too small. The degree of homogeneity of
the residual rays depends not only on the number of reflective surfaces, but
also on the intensity distribution in the spectrum of the incident radiation.
The wavelength, corresponding to the maximum of the isolated radiation,
also depends, on these circumstances, since i differs from unity.
A comparison of the behavior of the reflecting power for the series of
crystals usually employed at the present time as reflectors in the dbscribed
method* is given in Figure 3, which is taken from work [13].
To the data of the Figure, which represents a selection of crystals suitable
for the "residuaPrays" method, we should add some information on the reflec-
tive power of crystalline InSb [17] and KRS =5 [44] (a mixed crystal of T11 +
TIBr)..I4The first of these has a sharp maximum with 90% reflection at 54.6.1
the second a flatter peak (75%) at 200p. Suitable crystals for the longer-wave
region have, not yet been found.
In instruments designed for isolation of long-wav.e radiation by the "resid-
ual rays" method (diagrams of which are shown in Fig. 4), use-is usually made
*Information on the reflecting power of other crystals, and also?data on the
properties of various materials in the region of longer wavelengths may be
found in a survey of early works on long-wave IR spectroscopy, carried out
by Weniger [14] in 1923, and also in the monographs of Shaefer and Matossi
[15] and Parodi [16].
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i
15 00 IS X 35 00 45 51 55 0 21 115 10 80 A7 gm
/50
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Nal
lea
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Fig. 3. Reflection from surfaces of crystals [13].
*7W
Parodies
Parodies scheme Strong's scheme
4 4 4
te
.11
Fig. 4. Method-of residual rays [16]; MI, M2; M3, M4 ?
crystals, A, B, C, D, E ? spherical mirrors, R and S ?
receiver and source of radiation,
221p
of three or four reflections from plane crystal surfaces. Here the half-width
of the spectral portions isolated, e.g. for the case of crystals of NaC1 (reflec-
tion.peak at 52p), KC1 (62p) and KBr (83p) is equal to 43, 39 and 24 crn-I
respectively [12].
It is possible to design a scheme which would permit a much greater
number of reflections to be produced from the surface of the same crystal
and thus to increase several times the degree of rlionochroniatization (purity)
of the isolated radiation. Such schemes could be based on the methods of
multiple reflections.propoied by White [18], and also by Bernstein and Herz-
, .
berg [19] for obtaining long optical paths in gas cells of relatively smaU size.
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1." 40"
'
A diagram of an Instrument constructed in accordauce with White's
method is shown in Figure 5. Two identical concave spherical mirrors A1
and A.1 and a rectangular crystalline plate with a concave spherical surface
B. having the same radii of curvature, are placed so that the center of curva-.
wture lot the plate lies between the mirrors Ai and Ag, the centers of curve-
-
.. tuts al and aa of which musein turn lie symmetrically, at a certain distance
from sack Other, on the spherical surface of plate B. Then the radiation
catering the iastruMent through the entrance slit Spy/ill be reflected several
times and will be focused on the surface of the crystalline plate B before it
leaves through the exit slit Sa of the instrument.
Fig. 5. Obtaining multiple reflections from a crystal.
k is easy to see from FIgure 5 that the number of reflections fiorn the
caystal (ated's slumber of intermediate images of the entrance slit Si on its
earisco) is always odd and is equal to
a mt -4-- 1,
where d is the distance between the entrance and exit slits and Is the dis-
tance between the centers of curvature of the mirrors Al and Aa.
Thus tor a fixed 4 the number of reflections, and hence also the degree
of daelaschresnatlaation of the isolated radiation will depend only CM the &dile
411 of the mirrors As and A.1 round their common center b. With r
dilleaCielely =MU ((Siam) sad the actual dimensions of the crystalline plate
(S., X 10 X Ilnana) we can apparently obtain with this arrangement more than
Ii reflections. A change to other wavelengths can easily be accomplished by
,
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5.
,
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substituting plates made from different cyrstals (according to the data of
Fig. 3) which are fixed to the common movable stage.*
The advantage of the 'residual rays" method lies in the fact that by
comparatively simple means it enables the production of long-Wave radiation
of high intensity and of relatively high homogeneity. Drawbacks of the method
are that the limit of Its application in the long-wave region lies at 200p, alid
the fact that it is impossible to isolate a desired wavelength from a continuous
spectrum. Nonetheless this method is used at the present time not only for
obtaining information about the transmittance and reflecting power of various
materials in the long-wave IR region [15], but also for studying.the funda-
mental vibration frequencies of certain organic and inorganic substances, e.g.
cis- and trans-dichloroethylene, 1,2 - dichloroethane, methyl and propyl
alcohol, acetaldehyde, n-hexane, n-pentanc and carbon suboxide (C,0) [12J.
4. Monochromatization by means of diffraction gratings.
Great successes in the -analysis of infrared radiation have been achieved
with the help of diffraction gratings, the application of which alone permits -
the attainment of high resolution in the long-wave region of the spectrum.
o5l000e
?
A
19 7,17rTLTI.Ttl,-r,7TTZ7.7.75A'I
8)
Fig. 6. Profiles of diffraction gratings: a) wire, b) laminar,
c) echelette.
*Note that when using crystalline plates in the lsonger wavelength region there
i.a.,no necessity to prepare them with an accurately polNhed spherical surface
since a mat surface scatters short-wave radiation and is a good reflector for
long waves; besides this, such plates can apparently be prepared by pressing
or by deposition of crystalline layers on a spherical glass surface by evapora-
tion of the substance in vacuo or by precipitation from solutions.
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..
Prisms cannot be used since at present there are no materials which in thick
layers are sufficiently transparent in the wavelength region > 5011 and which
are suitable for making prisms.*
In early investigations of long-wave IR sPectra transmission wire [21]
and reflection laminar (plate) [22] gratings Were employed (Fig. 6, a and b).
In recent years and at the present time exclusive use is made of reflection
stepped echelette gratings [23] which, because of the. specially chosen groove
profile (Fig. 6, c) are capable of concentrating the greater part of the incident
energy in a narrow region of diffraction angles, for example, in the region of
one of the first orders. In the most favorable conditions the intensities of
long-wave radiation isolated by using wire, laminar and stepped (echelette)
gratings are in the ratio 1:4:10 - - I.
The wavelength distribution . e spectrum given by reflection gratings
is described by the known equation
ml = d (sin 4, + sin
where m ? spectral order, d? grating constant, and 4 and i ? angles of inci-
dence and diffraction.**
If the spectrum is scanned by rotating the grating at fixed angles of inci-
dence and reflection, which is the usual procedure in spectrometry, the wave-
length will be
mA.F-- 2d cos Te sib /I
or mA =K sin ft, where 0 ?angle between the directions Of incidence and dif-
fraction, 13? angle of rotation of grating (Fig. 7) and K? instrument constant.
? The relative intensity distribution, for example in the first order spec-
-
trum of the echelette, which depends on the wavelength and the groove form
of the echelette surface, is determined from the expression
*As early as 1898 4ubens and Aschkinass [20] made an attempt to isolate
long-wave radiation by using acute-angled quartz prisms. Yet the result was
unsatisfactory, since they did not succeed in removing the short-wave radia-
tion to the desired extent.
**In this formula the (+)sign corresponds to the case where the incident and -
diffracted rays lie on one side of the normal to the grating, and the (?)sign
when these rays lie on different sides of it.
" " ?
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